Transparent synthetic linear polyamide and process for producing same



United States Patent'D TRANSPARENT SYNTHETIC LINEAR POLY- AND PRGCESS FOR PRODUCING SAME Donald C.v Pease, Wilmington, Del., assignor to E. I. du

Pont de Nemours and Company, Wilmington, Del, a corporation of Delaware No Drawing. Application July 19, 1951, Serial No. 237,678

3 Claims. :(cr. 260-78) .additives are necessary with most copolyamides to obtain clear products, particularly in'the form of moldings as contrasted to the'fibers. Although polyamides generally have high melting points, for example l60240 C., molded objects begin to lose their rigidity at temperatures of the order of 100-125 C.

Transparent polyamides suitable for molding into products of high strength and dimensional stability have been provided by the process of U. S. 2,512,606, wherein a normally liquid isomer mixture of bis(4-aminocyclohexyl)methane, equally correctly termed di(p-aminocy clohexyl) methane,

CH2CH2 GET-(3H2 is reacted with a straight chain alpha, omega-alkanedioic acid of 6-10 carbons. These polyamides are transparent and moldable, but they are, in general, not dimensionally stable at temperatures above 100 C. and lose their strength at high temperatures, e. g., 200 C. or higher.

There has now been found a new polyamide having the recurring unit,

of the carbons attached to the bridge methylene and the carbons attached to the amino groups with respect to the cyclohexyl ring (i. e., the cis and trans configurations of the carbon-hydrogen bonds of those carbons in the above formula which have only one hydrogen). The three expected isomers differ in physical properties, particularly in melting points. The above diamine is obtained as a mixture of isomers by hydrogenation of di(p-aminophenyl)rnethane. This latter compound results from the reaction of formaldehyde with aniline. Depending upon the choice of conditions for the hydrogenation either a mixture of stereoisomers is obtained that is liquid at 25 'C., i. e.;normally'liquid, or-a-mixtureisobtained thatis s'olidat'25 C., i. e., normally solid,'the latter mixture generally melting above 40 C.

'The preparation and ipropertiesof the normally-liquid mixture of stereoisomers of 'di(p-aminocyclohe xyl)- methane are fully disclosed in U. S- application Serial No.

98,174, filed 'June' 9, '1949 in the name of G. M. Whitman and now Patent Number 2,606,924. In. general, the preparation of-normally liquid mixtures-of'stereoisomers is favoredbythe-use of a ruthenium hydrogenation catalyst at' relativelylow temperatures whereas higher temperatures and other hydrogenation catalysts favor the production of the higher melting-normally solid mixtures of stereoisomers of di(p-aminocyclohexyl)methane.

The properties of the new polyamide ofthis invention are quite unexpected and were unpredictable. As shown in U. S. 2,516,585, terephthalic acid' gave with'the liquid isomer mixture, di(p-aminocyclohexyl)methane, an

opaque polymer which was infusible at temperatures as meta isomer,' isophtha'lic acid, gives withthe samediamine a transparent polyamide which is thermoplastic, retains'its' stitfness'and molded'dimensions at temperaturesof at least 200 C. I

The followingexam'ples 'in' which the parts are' by ---weight'='illustrate' specific embodiments'of this invention.

Example 1.To 297.2 parts of bis(4-aminocyclohexyl)methane, which is a mixture of isomers that is liquid at 25 C. (obtained by the hydrogenation of di)paminophenyl)methane with ruthenium catalyst at C.), in a glass-lined pressure-resistant reactor was added 449.7 parts of diphenyl isophthalate. The reactor was purged with oxygen-free nitrogen and heated for five minutes at 255 C. under nitrogen at a pressure of one atmosphere. Heating under nitrogen was continued at 306 C. for fifteen minutes and at 330 C. for fifteen minutes while phenol liberated from the reaction was distilled. The reaction was completed in fifteen minutes at 330 C. under a pressure of 5 mm. of mercury. The polymer had an inherent viscosity of 1.12 (the 111 relative viscosity of an 0.5% solution in mcresol at 25 C. divided by 0.5). Transparent colorless films were obtained by heating this polymer under a pressure of 2-3000 lb./ sq. in. at 330 C. The film had a stiffness of 370,000 lb./sq. in. at 25 C. and 50% relative humidity and 350,000 lb./ sq. in. after soaking in water at 25 C. for seventy-two hours. The film had a stiffness of 210,000 lb./sq. in. at 200 C. These films retained their original dimensions and optical clarity after exposure to boiling water for several days. The polymer had a water absorption to saturation of 7.75%.

Example 2.-To 882 parts of bis(4-aminocyclohexyl)methane, described in Example 1, in a glass-lined, pressure-resistant reactor was added 696.5 parts of isophthalic acid and 1000 parts of phenol. The reactor was purged with oxygen-free nitrogen, sealed and heated for one hour at 210 C. under autogenous pressure. The reactor was opened and the contents heated at reflux for one-half hour under nitrogen at one atmosphere pressure. The reactor was then heated one hour at 285 C. while phenol and water from the condensation reaction distilled. Heating was continued for two hours at 285 C. and one-half hour at 306 C. under a pressure of 5 mm. of mercury. Colorless, transparent polymer was obtained which has an inherent viscosity of 0.89.

Example 3.-To 207.5 parts of bis(4-aminocyclohexyl) methane described in Example 1, mixed with 200 parts of boiling water was added 163.9 parts of isophthalic acid. The mixture was boiled for three minutes and filtered. To the filtrate was added 3500 parts of absolute ethyl alcohol. The mixture was cooled to 20 C. and in 48 hours salt amounting to 200 parts was separated by filtration. Fifty parts of this salt was polymerized with 50 parts of phenol in a way similar to that described in Example 2; Polymer with an inherent viscosity of 0.85 was obtained.

The examples show the equivalence of isophthalic acid and its esters in the amide-forming reaction. In carrying out the polyamide-forming reaction isophthalic acid or its suitable amide-forming derivatives is heated with a normally liquid mixture of bis(4-aminocyclohexyl)methane in substantially molar amounts, that is, in molar ratios that do not depart more than about 0.5% from equimolecular amounts. A very slight excess of one of the reactants can be present to control molecular weights within desired limits. Optimum rates of polymerization are obtained by heating the amide-forming ingredients to at least 200 C. and below 350 C., and preferably in the range of 250-330 C., although, as noted hereinabove, the polymerization takes place at temperatures as low as 100 C., or even lower. The heating is generally under an inert atmosphere.

Although up to about 5% of other polyamide-forming materials may be present, the optimum properties are obtained with the diamine and dibasic acid as shown in the examples.

The polyamide of this invention has an inherent viscosity of at least 0.4 and generally at least 0.6, and loses rigidity of molded objects at a temperature that approaches the melting point of the polyamide. This high a softening point, that is, 200 C. or higher, is rare in thermoplastics and even exceeds the distortion temperature of many thermosetting resins. In addition to this unusual and unexpected property the polymer is substantially unaffected by boiling water, even after several days exposure. transparent, and is easy to mold or extrude. This novel combination of properties makes the polyamide useful in the preparation of molded objects, bristles, and fibers. Particularly important are uses where rigidity at elevated temperature is required, such as for electrical applications The polymer possesses superior stiffness, is

4 or in bristle applications where the bristle is subjected to superheated steam.

I claim:

1. A process for making a transparent polyamide which comprises heating, in a molar ratio which is Within 0.5% of equimolar amounts, a mixture of isomeric bis(4-aminocyclohexyl)methanes which is liquid at 25 C., and an isophthalic acid compound of the class consisting of isophthalic acid and amide-forming derivatives thereof, at a temperature of from 100 C. to 350 C. whereby a transparent polyamide in which the polyamide-forming ingredients consist essentially of the said isophthalic acid compound and the said isomeric bis(4-aminocyclohexyl) methanes is obtained.

2. The process set forth in claim 1 in which the said temperature is from 250 to 350 C.

3. The transparent polyamide, obtained according to the process of claim 1, which consists, in major amount, of recurring structural units of the formula CH2-CH2 CHE-CH2 -HNCH H-O-OHrC-H HCNHC -og H H CHrC 2 CHz-OH2 O 0 References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,244,192 Flory June 3, 1941 2,252,554 Carothers Aug. 12, 1941 2,512,606 Bolton et al. June 27, 1950 2,516,585 Pease July 25, 1950 2,606,924 Whitman Aug. 12, 1952 

1. A PROCESS FOR MAKING A TRANSPARENT POLYAMIDE WHICH COMPRISES HEATING, IN A MOLAR RATIO WHICH IS WITHIN 0.5% OF EQUIMOLAR AMOUNTS, A MIXTURE OF ISOMERIC BAS(4-AMINOCYCLOHEXYL) METHANES WHICH IS LIQUID AT 25* C., AND AN ISOPHTHALIC ACID COMPOUND OF THE CLASS CONSISTING OF ISOPHTHALIC ACID AND AMIDE-FORMING DERIVATIVES THEREOF, AT A TEMPERATURE OF FROM 100* C. TO 350* C. WHEREBY A TRANSPARENT POLYAMIDE IN WHICH THE POLYAMIDE-FORMING INGREDIENTS CONSIST ESSENTIALLY OF THE SAID ISOPHTHALIC ACID COMPOUND AND THE SAID ISOMERIC BIS(4-AMINOCYCLOHEXYL) METHANES IS OBTAINED. 